1. Field of the Invention
The present invention relates to a wireless communication system and, more particularly, to an apparatus and a method of transmitting and receiving data in soft handoff of a wireless communication system.
2. Discussion of the Related Art
First of all, a related art power control technique will be described.
Power control in a mobile communication system refers to a technique controlling the power level of a receiving signal in a receiving end, so that the system can be operated to a level required by modulating and coding methods that are adopted according to a transmission rate of transmission data. Particularly, power control relates to resolving a near-far problem that occurs in a reverse link. More specifically, by controlling transmission power of mobile stations so that the transmission power of a mobile station nearer (or closer) to the base station is different from a mobile station further from the base station, the power level of each mobile station received by the base station may be controlled to a specific level.
The mobile station transmits a power signal along with a data signal through a reverse link. Herein, reverse power control is performed by controlling transmission power of a mobile station in a way that the receiving energy of a reverse pilot channel (R-PICH) can be constant. A receiving end of the base station measures the receiving energy of the reverse pilot channel. Then, when the receiving energy is higher than a setpoint, which is a predetermined reference energy level, the receiving end of the base station transmits a DOWN power control bit (PCB) command, which means to lower the transmission power, to the mobile station. And, when the receiving energy is lower than the setpoint, the receiving end of the base station transmits an UP power control bit (PCB) command, which means to increase the transmission power, to the mobile station through a forward power control subchannel (F-PCSCH).
Based upon such pilot channel power control, the power control of a reverse traffic channel (R-TCH), wherein data are transmitted through a reverse link, is performed. More specifically, the transmission power of the reverse traffic channel is decided by using a ratio between the transmission power of a pilot channel and the transmission power of a traffic channel (traffic to power ratio, TPR). The TPR for each data rate of the data being transmitted through the traffic channel is pre-decided, and the transmission power of the pilot channel varies in PCB units based upon the reverse link power control. Thus, the relation between the variable transmission power of the pilot channel and the pre-decided TPR decides the transmission power of the traffic channel through which data are transmitted.
Hereinafter, a hybrid automatic repeat request (hereinafter referred to as “HARQ”) method and an early termination technique of the related art packet data will now be described in detail.
The HARQ method, which is used in order to enhance the transmission efficiency of packet data that have properties less sensitive to transmission delay, consists of a combination of a conventional forward error correction (hereinafter referred to as “FEC”) method and an automatic repeat request (ARQ) method through error detection. The HARQ method is used in connection with a physical layer, and the HARQ method combines the retransmitted data with previously received data, thereby ensuring a high decoding success rate. More specifically, the HARQ method corresponds to a method that stores that has failed to be transmitted, instead of discarding the transmission-failed packet, which is then combined with the retransmitted data, thereby being decoded.
According to the HARQ method, the transmitting end FEC-codes data packet information, so as to divide the coded bits into a plurality of sub-packets and transmit the sub-packets. A sub-packet may decoded by using a single sub-packet and has a structure indicating transmission success/failure. Also, the receiving end combines the sub-packet of an identical packet previously received with the currently received sub-packets and decoded the combined packets, so as to verify the success or failure of the transmission. After the transmission of the first sub-packet, the transmitting end receives an acknowledgement (ACK/NACK) from the received end. Then, if the acknowledgement is a negative acknowledgement (hereinafter referred to as “NACK”), another sub-packet is additionally transmitted. And, if the acknowledgement is an affirmative (or positive) acknowledgement (hereinafter referred to as “ACK”), the transmission of the corresponding packet is ended.
In case N number of sub-packets is generated by using the HARQ method, when the transmitting end transmits an Mth sub-packet (M<N) and receives an ACK feedback, the transmitting end ends the transmission of the corresponding packet without transmitting the remaining sub-packets. This method is referred to as an early termination method. When using the early termination method, since unnecessary sub-packets are not transmitted, the packet transmission efficiency may be largely enhanced.
Hereinafter, an early termination method of a related art circuit channel will now be described in detail.
A voice (or audio) service being sensitive to transmission delay and generating contiguous data is transmitted through a circuit channel. The circuit channel is a form of channel that performs data transmission without interruption.
FIG. 1 illustrates a method of controlling power of a reverse link in a CDMA 2000 system.
As shown in FIG. 1, in the code division multiple access (CDMA) 2000 system, which is currently being widely used, a frame of a traffic channel having the structure of a circuit channel generally corresponds to 20 ms. Each frame includes 16 slots, and each slot corresponds to 1.25 ms. Since the receiving end transmits one PCB for each slot, each slot is referred to as a power control group (PCG).
In order to enhance the transmission efficiency of the traffic channel having the circuit channel structure, the organization for the standardization of the 3rd Generation Partnership Project2 (3GPP2) is currently debating on whether or not to apply the early termination method in circuit channels. Unlike the conventional method of having the receiving end receive a whole frame of 20 ms and then decoding the data, the early termination method in circuit channels attempts to decode data during the reception of the frame. Accordingly, if the data reception is successfully completed, the transmitting end sends an ACK feedback, thereby interrupting the transmission of the corresponding frame. Since this method interrupts (or discontinues) unnecessary transmission in the CDMA system, interference with other users can be reduced. Thus, the overall system capacity (or size) may be increased.
FIG. 2 illustrates an example of applying the early termination method in a reverse link traffic channel. Referring to FIG. 2, the base station attempts to decode data during the reception of a frame. Then, when data are successfully received, the base station transmits an ACK feedback to the mobile station through a forward acknowledge subchannel (F-ACKSCH). Then, once the ACK is received, the mobile station discontinues transmission of the corresponding frame.
In a soft handover, a mobile station receives traffic channels carrying identical information from two or more base stations. Then, the mobile station demodulates the traffic channels received from each of the two or more base stations and combines the demodulated traffic channels, thereby performing decoding.
In the CDMA system, the traffic channels transmitted by a single base station are differentiated (or identified) by a spread code. More specifically, the base station allocates different walsh codes to each of the traffic channels. Then, the base station modulates the signal by using the allocated walsh codes, thereby transmitting the modulated signal. At this point, in order to transmit a plurality of traffic channels without interference among one another in a single base station, the coding rate for the FEC channel coding in the traffic channel should be should be high. In the CDMA 2000 system, in case of radio configuration 4, the coding rate for the FEC channel coding in traffic channel of a forward link is ½.
When the coding rate is ½, when the early termination method is applied, decoding may be successful at the decoding attempt point only when at least half or more of a 20 ms frame has been received. In other words, the decoding success rate when less than half of a frame is received is equal to 0.
Therefore, there lies a problem in that the gain of the early termination method cannot be enhanced.